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/* Copyright (C) 2009, 2011 Free Software Foundation, Inc.
   Contributed by Richard Henderson <rth@redhat.com>.
 
   This file is part of the GNU Transactional Memory Library (libitm).
 
   Libitm is free software; you can redistribute it and/or modify it
   under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.
 
   Libitm is distributed in the hope that it will be useful, but WITHOUT ANY
   WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
   FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
   more details.
 
   Under Section 7 of GPL version 3, you are granted additional
   permissions described in the GCC Runtime Library Exception, version
   3.1, as published by the Free Software Foundation.
 
   You should have received a copy of the GNU General Public License and
   a copy of the GCC Runtime Library Exception along with this program;
   see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
   <http://www.gnu.org/licenses/>.  */
 
#ifndef GTM_RWLOCK_H
#define GTM_RWLOCK_H
 
#include <pthread.h>
#include "local_atomic"
 
namespace GTM HIDDEN {
 
struct gtm_thread;
 
// This datastructure is the blocking, mutex-based side of the Dekker-style
// reader-writer lock used to provide mutual exclusion between active and
// serial transactions. It has similarities to POSIX pthread_rwlock_t except
// that we also provide for upgrading a reader->writer lock, with a
// positive indication of failure (another writer acquired the lock
// before we were able to acquire). While the writer flag (a_writer below) is
// global and protected by the mutex, there are per-transaction reader flags,
// which are stored in a transaction's shared state.
// See libitm's documentation for further details.
//
// In this implementation, writers are given highest priority access but
// read-to-write upgrades do not have a higher priority than writers.
 
class gtm_rwlock
{
  pthread_mutex_t mutex;	        // Held if manipulating any field.
  pthread_cond_t c_readers;	        // Readers wait here
  pthread_cond_t c_writers;	        // Writers wait here for writers
  pthread_cond_t c_confirmed_writers;	// Writers wait here for readers
 
  static const unsigned a_writer  = 1;	// An active writer.
  static const unsigned w_writer  = 2;	// The w_writers field != 0
  static const unsigned w_reader  = 4;  // The w_readers field != 0
 
  std::atomic<unsigned int> summary;	// Bitmask of the above.
  unsigned int a_readers;	// Nr active readers as observed by a writer
  unsigned int w_readers;	// Nr waiting readers
  unsigned int w_writers;	// Nr waiting writers
 
 public:
  gtm_rwlock();
  ~gtm_rwlock();
 
  void read_lock (gtm_thread *tx);
  void read_unlock (gtm_thread *tx);
 
  void write_lock ();
  void write_unlock ();
 
  bool write_upgrade (gtm_thread *tx);
  void write_upgrade_finish (gtm_thread *tx);
 
 protected:
  bool write_lock_generic (gtm_thread *tx);
};
 
} // namespace GTM
 
#endif // GTM_RWLOCK_H